(1) Field of the Invention
The present invention relates to solid-state imaging devices and manufacturing methods thereof, and in particular to a solid-state imaging device including light-receiving units arranged in rows and columns and a vertical charge transfer unit which transfers signal charge obtained as a result of conversion by the light-receiving units.
(2) Description of the Related Art
Solid-state imaging devices represented by charge-coupled device (CCD) image sensors are widely used as imaging elements of imaging apparatuses such as digital still cameras and digital video cameras, and are in increasing demand. With the recent shift to the high-definition television, imaging apparatuses are now required to handle high-definition videos. This means that higher transfer frequency is required of solid-state imaging devices.
As a technique to enable the higher transfer frequency, that is, high-speed transfer, it is known to connect shunt wires and transfer electrodes in a vertical charge transfer unit via vertically extending light-shielding films (refer to Patent Reference 1: Japanese Unexamined Patent Application Publication No. 04-279059, for example).
However, vertical deposition of shunt wires as shown in Patent Reference 1 does not mean that voltages of the same level are applied simultaneously to all the light-shielding films connected to the shunt wires. As a result, voltages of different levels are applied to adjacent light-shielding films.
The light-shielding films are deposited for each of columns of vertically arranged pixels. This creates a difference, between adjacent columns of pixels, in level of voltages applied between the light-shielding films and the upper-side interface of the semiconductor substrate. As a result, the solid-state imaging device of Patent Reference 1 has a difference, between the adjacent columns of pixels, also in amount of charge captured and lost in the interface state of the semiconductor substrate while the charge is read out from the light-receiving units to the vertical charge transfer unit.
Due to these differences, the conventional solid-state imaging device has a problem of unevenness in the output.
To solve this problem, a technique of horizontally forming shunt wires is known (refer to Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2006-41369, for example).
Hereinafter, a conventional solid-state imaging device of Patent Reference 2 is described with reference to FIGS. 7A to 7C.
FIG. 7A is a plan view of light-receiving units 111 and vertical charge transfer units 113 of a conventional solid-state imaging device 100. FIG. 7B is a cross-sectional view showing the Y0-Y1 plane of FIG. 7A. FIG. 7C is a cross-sectional view showing the X0-X1 plane of FIG. 7A.
The conventional solid-state imaging device 100 includes a plurality of light-receiving units 111 arranged in rows and columns and a plurality of vertical charge transfer units 113 provided for each column. Each of the vertical charge transfer units 113 transfers, in the vertical direction (the column direction), signal charge obtained as a result of photoelectric conversion by the light-receiving units 111 arranged in a corresponding column, and then outputs the signal charge to a horizontal charge transfer unit (not shown). It is to be noted that the lateral direction of FIG. 7A is referred to as the vertical direction and the longitudinal direction of FIG. 7A is referred to as the horizontal direction.
Each of the vertical charge transfer units 113 includes a transfer channel 112, insulating films 102, 105, and 108, a plurality of first transfer electrodes 103a, a plurality of second transfer electrodes 103b, insulating regions 104, shunt wires 107a and 107b, a light-shielding film 109, and contacts 110a and 110b. 
The transfer channel 112 extends vertically, and is horizontally coupled to the light-receiving units 111 arranged in the corresponding column.
The first transfer electrodes 103a and the second transfer electrodes 103b are formed in the same layer, and are deposited above the transfer channel 112. One first transfer electrode 103a and one second transfer electrode 103b are deposited for each light-receiving unit 111. The first transfer electrodes 103a and the second transfer electrodes 103b are deposited alternately in the vertical direction.
Each of the insulating regions 104 is formed between one first transfer electrode 103a and one second transfer electrode 103b above the transfer channel 112 to insulate the first transfer electrode 103a and the second transfer electrode 103a. The width of each insulating region 104 is between 0.05 μm and 0.15 μm inclusive approximately, to prevent transfer troubles.
The shunt wires 107a and 107b are formed above the first transfer electrodes 103a and the second transfer electrodes 103b. The shunt wires 107a correspond one-to-one with the first transfer electrodes 103a, whereas the shunt wires 107b correspond one-to-one with the second transfer electrodes 103b. The shunt wires 107a are electrically connected to the first transfer electrodes 103a via the contacts 110a. The shunt wires 107b are electrically connected to the second transfer electrodes 103b via the contacts 110b. The shunt wires 107a and 107b have resistance lower than that of the first transfer electrodes 103a and the second transfer electrodes 103b. For example, the shunt wires 107a and 107b are made of a metal such as tungsten, whereas the first transfer electrodes 103a and the second transfer electrodes 103b are made of polysilicon.
The insulating film 105 is formed between the shunt wires 107a and 107b and the first transfer electrodes 103a and the second transfer electrodes 103b. 
The insulating film 108 is formed over the shunt wires 107a and 107b. 
The light-shielding film 109 is formed on the insulating film 108. The light-shielding film 109 has an opening 114 above each of the light-receiving units 111.
Here, in the case of arranging pixels of about 2 μm square, the width W1 of a section of the first transfer electrodes 103a extending horizontally between the light-receiving units 111 is 0.45 μm approximately. Further, the total number of shunt wires 107a and 107b deposited in each row is equal to the number of transfer electrodes deposited for one light-receiving unit 111, which is two in this example. The width W2 of the shunt wires 107a and 107b is 0.12 μm, for example, and the space W3 between each shunt wire 107a and an adjacent shunt wire 107b is 0.16 μm, for example.
Next, a method for manufacturing the conventional solid-state imaging device 100 is described.
First, as shown in FIGS. 7B and 7C, the insulating film 102 is formed on the surface of a semiconductor substrate 101 by a thermal oxidation method. Then, formation of various resist patterns and ion implantation are performed on the semiconductor substrate 101. By doing so, the light-receiving units 111 and the transfer channel 112 are formed.
Next, after forming a conductive film such as a polysilicon film, the conductive film on the transfer channel is divided to form the first transfer electrodes 103a and the second transfer electrodes 103b. Here, as described above, since the insulating regions 104 need to be formed with widths between 0.05 μm and 0.15 μm inclusive approximately, the insulating film 105, which is to be used as a hard mask, is generally first deposited on the conductive film using a method such as CVD.
To be more specific, after forming the insulating film 105 on the entire surface, a resist pattern having widths between 0.15 μm and 0.30 μm inclusive approximately is formed on the insulating film 105 using a photolithographic method. Next, grooves are formed on the insulating film 105 using anisotropic etching using the resist pattern. Then, an oxide film of 0.05 μm to 0.10 μm in thickness is deposited on the side surfaces of the grooves using a method such as CVD so as to form side walls on the side surfaces of the grooves. By doing so, the grooves are narrowed to a desired width.
Next, the anisotropic etching is performed using, as a hard mask, the insulating film 105 having the grooves, so that grooves which vertically divide the conductive film are formed.
Next, the conductive film in a region other than the transfer channel 112 is etched using the photolithographic method, so that the first transfer electrodes 103a and the second transfer electrodes 103b are formed.
Next, film formation using a method such as CVD forms the insulating regions 104 to fill the isolate regions (grooves) between the first transfer electrodes 103a and the second transfer electrodes 103b above the transfer channel 112.
Next, the contacts 110a and 110b are formed to penetrate the insulating film 105. Next, a conductive film is formed to entirely cover the upper surface of the semiconductor substrate 101, including over the first transfer electrodes 103a and the second transfer electrodes 103b. Specifically, for example, a thin metal film made of tungsten or the like is formed using a method such as CVD or sputtering.
Next, the anisotropic etching is performed on the thin metal film using the photolithographic method, so that the shunt wires 107a and 107b are formed. Here, to allow the space W3 between each shunt wire 107a and the adjacent shunt wire 107b to be 0.16 μm approximately, the etching is performed at a low selective ratio to make dimensional loss small.
Next, the insulating film 108 is deposited by a method such as CVD. Subsequently, a light-shielding metal film is deposited on the insulating film 108 and is then etched using the photolithographic method, so that the light-shielding film 109 is formed.
The above processing makes the structure shown in FIGS. 7A to 7C.
After that, lens elements are so on are formed if necessary (not shown).
With the above structure of the conventional solid-state imaging device 100, the shunt wires 107a and 107b are not electrically connected with the light-shielding film 109, thereby allowing the levels of voltages applied between the light-shielding film 109 and the upper-side interface of the semiconductor substrate 101 to be uniform among the pixel columns. As a result, the solid-state imaging device 100 can reduce the unevenness in the output.